# Questions tagged [kerr-metric]

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### What happens when two ergospheres of oppositely spinning black holes of the same mass, axis of rotation, and speed of rotation approach?

What happens when two ergospheres of oppositely spinning black holes of the same mass, axis of rotation, and speed of rotation approach each other? What would happen to the black holes in question?
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### Gravitational Redshift in Kerr Spacetime

Well, suppose then Schwarschild black holes. Following the $$, we have the redshift factor: $$d\tau = \sqrt{1-\frac{2M}{r}}dt. \tag{1}$$ This factor have an physical interpretation to be the ...
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### Spin rotation of masses in a rotating gravitational field around a black hole

If a rotational gravitational field affects masses in the way the frame and body velocities are added together does it mean that as the rotation of the frame has a gradient of the perpendicular ...
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### Science of Kerr Black holes, can naked singularity form? Does it lead to another universe?

In the science fiction series, XeeLee Sequence, Stephen Baxter had used quite a lot of accurate physics in this hard science fiction. I was able to follow and determined which ones are true of our ...
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### Horizons and other special surfaces on Kerr metric

The Kerr metric is \begin{equation} ds^2 = - \big(1-\frac{2GMr}{\rho^2}\big)dt^2-\frac{2GMra}{\rho^2}\sin^2 \theta \big(dtd\phi+d\phi dt\big)+ \frac{\rho^2}{\Delta}dr^2+\rho^2 d\theta^2 + \frac{\sin^...
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### Trying to determine physically meaningful initial conditions for test particle around a Kerr black hole

I am working on some code for a test particle orbiting a Kerr black hole. I am trying to determine what the initial $\frac{d t}{d\tau}$ would be. On Wikipedia https://en.wikipedia.org/wiki/...
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### Symmetries of quasinormal modes of Kerr black hole

I have been reading this following paper on numerical evolution of the Teukolsky equation (see e.g. Eq 1 in their paper) for spin -2 fields about a spinning black hole (Kerr) solution. As the ...
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### Gravitational waves from collapsing rotating mass? [closed]

According to the Birkhoff’s theorem, a non-rotating mass does not radiate gravitational waves during a spherically symmetric collapse. There is, however, no equivalent of the Birkhoff’s theorem for a ...
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### Why do angular velocity deviates from Keplerian value close to a black hole?

The energy-momentum conservation equation in General Relativity is expressed as $T^{\mu\nu}_{\;\;;\nu}=0$ and by projecting this equation into the space normal to the four-velocity, we obtain the ...
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### Does matter take infinite time to leave a black hole?

To this question: Can a sufficiently large black hole be singularity-free? about the possibility of a sufficiently large black hole not containing a singularity John Rennie posted an answer ...
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### Time Derivative defined by Fermi transport in Kerr Space-Time

I have been studying 3+1 GR the last weeks. I have tried to comprehend Electrodynamics in curved space-time: 3+1 formulation (1982) Thorne McDonald I have studied another 3+1 GR book but it didn'...
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### What is exactly rotating in a rotating black hole?

I have read this: https://arxiv.org/abs/gr-qc/9404041 In General Relativity the black hole solutions which have so far been found form a four parameter family called the generalized Kerr-...
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### How to derive the angular velocity of circular orbits in Kerr geometry?

I am trying to derive the angular velocity of a circular orbit in Kerr geometry, eqn.(2.16) in Bardeen et al (1972) which reads $$\Omega=\dfrac{1}{r^{3/2}+a}$$ (Note that I am using the units in which ...
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### Metric for a rotating star

If we want to describe a static spherically symmetric star we can use a metric which matches the Schwarzschild solution with correct mass on the outside of the star but differs from Schwartzschild in ...
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### Light-like normal vectors

Can someone please show me how to mathematically establish that the normal vector to the event horizon of a Kerr Black Hole is light-like?
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### Numerical Solutions for Equatorial Orbits in the Kerr Black Hole

Currently, I am trying to find timelike orbits in the Kerr metric around the equator. The problem is that no matter which parameters I choose or the method I use I can't seem to get to physically ...
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### How to compute Kerr geodesics?

How would I start to numerically compute trajectories of Kerr geodesics with constants of motion like in this wikipedia page. I want to recreate trajectories like in this picture in Matlab.
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### How does the Penrose diagram for a spinning black hole differ in realistic scenarios (formed by stellar collapse)?

The Penrose diagram for a non-spinning Schwarzschild black hole is Notably, there is a second universe "on the other side" of the black hole. However, actual black holes form by stellar collapse, and ...
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### Metric diameter of a ring singularity

In the Kerr metric the ring singularity is located at the coordinate radius $r=0$, which corresponds to a ring with the cartesian radius $R=a$. So the center of the ring singularity in cartesian ...
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### Quadrupole moment of Kerr spacetime

In this paper this paper, the Kerr black hole is described as having quadrupole moment of $q=J^2/M$ (which means $q=a^2M$ using $J=aM$) whereas in this paper it says in the abstract that the limiting ...
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### For a given mass, how big can a Kerr black hole get?

We know that in a Kerr Black Hole the singularity is in the form of a 1 dimensional ring. If we have a 25 solar mass black hole, how big would the Kerr Ring be, width wise? Also, I read the Wiki on ...
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### Centrifugal force on spinning black hole?

I saw the term spinning black hole popping up everywhere so my question do spinning black hole behave similarly to say a planet where it bulge in the equatorial and compress at the poles? what ...
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### Kerr Black hole EH and Ergosphere embedding

Goodmorning everyone. I would like to share with you a question that has been gripping me for some time, but which I have never been able to give a convincing answer. When representing the ergosphere ...
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### What would happen to the Earth, if the moon was a black hole? [closed]

Would it be a feasible scenario? I have read this question: What would happen to the Moon if Earth is turned into a black hole? Where Lubos Motl says: The extremal Kerr J=GM2/c∼RbhMc. Now, the ...
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### Periodicity trick for Kerr Black Holes

I am slightly confused concerning the euclidean section of a Kerr black hole. In page 5 of the following paper https://arxiv.org/abs/hep-th/9908022 it is said that in order to get the euclidean ...